"There is no reason to think that there is any substance to any of these charges," he told Al Jazeera from Melbourne.

"These are clearly political charges, which is an attempt by China to intimidate Australia, in which it has been largely successful."

Reporters outside the court said some journalists, mostly from Chinese state media, were allowed in to cover Monday's opening session.

However, China has said the hearings on the industrial secrets charges will be held behind closed doors, raising questions from Australian officials over whether the men will get a fair trial.

Australia has said its consular officials will attend trial sessions on the bribe-taking charges and it had asked China to reconsider the closure of the trade secrets hearings.

In a statement issue over the weekend, the Australian Department of Foreign Affairs and Trade said it was disappointed with the Chinese decision to bar access to some parts of trial.

Disappointment

"The Government's disappointment with the decision has been registered with Chinese officials in Beijing and Canberra," it said.

"The Australian Government does not propose to make further representations on this matter."

Rio Tinto has said its employees did nothing wrong [AFP]

The department said Hu's lawyer, Duan Qihua, will be present throughout the trial.

The four defendants were arrested last July during contentious iron ore contract negotiations which later collapsed, and after Rio snubbed a near $20bn cash injection from state-run Chinese mining firm Chinalco.

Rio Tinto, based in London and Australia, is the world's third-largest mining company and one of China's biggest suppliers of Iron ore, one of the main ingredients for making steel.

At the time of the executives' arrests Rio was acting as lead negotiator for global iron ore suppliers in price talks with Chinese steel mills.

The case comes at a time of friction with the US over China's currency policies and doubts among some in the foreign business community over Beijing's commitment to building an open and fair business environment.

Joerg Wuttke, the president of the European Union Chamber of Commerce in China, told the AFP new agency the trial was being seen as a barometer of legal transparency in China.

"It will be regarded as a litmus test of the status of the young Chinese legal system," he said.